linux-kernel - Re: [PATCH v3 1/3] sched: introduce a new migration flag to task

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Date:	Thu, 15 Oct 2015 21:26:44 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	yuyang.du@...el.com, pjt@...gle.com, efault@....de,
	tglx@...utronix.de
Subject: Re: [PATCH v3 1/3] sched: introduce a new migration flag to
 task_struct

On Thu, Oct 15, 2015 at 01:18:13PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 15, 2015 at 06:01:14PM +0900, byungchul.park@....com wrote:
> > From: Byungchul Park <byungchul.park@....com>
> > 
> > This patch removes a weird coupling between se->avg.last_update_time and
> > the condition checking for migration, and introduce a new migration flag.
> > Now, scheduler can use the flag instead of se->avg.last_update_time to
> > check if migration already happened or not.
> 
> Was there a problem with that coupling? This does not explain.

The reason why i introduce the new flag is that 3/3 patch makes
se->avg.last_update_time non-zero consistently, so we cannot use the
condition "se->avg.last_update_time == 0" to check if migration has
happened.

> > +++ b/kernel/sched/fair.c
> > @@ -2771,14 +2771,15 @@ static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
> >  
> >  /* Add the load generated by se into cfs_rq's load average */
> >  static inline void
> > -enqueue_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > +enqueue_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> >  {
> >  	struct sched_avg *sa = &se->avg;
> >  	u64 now = cfs_rq_clock_task(cfs_rq);
> > -	int migrated, decayed;
> > +	int decayed;
> > +	int migrated = flags & ENQUEUE_MIGRATED;
> > +	int created = !sa->last_update_time;
> >  
> > -	migrated = !sa->last_update_time;
> > -	if (!migrated) {
> > +	if (!migrated && !created) {
> >  		__update_load_avg(now, cpu_of(rq_of(cfs_rq)), sa,
> >  			se->on_rq * scale_load_down(se->load.weight),
> >  			cfs_rq->curr == se, NULL);
> > @@ -2789,10 +2790,10 @@ enqueue_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >  	cfs_rq->runnable_load_avg += sa->load_avg;
> >  	cfs_rq->runnable_load_sum += sa->load_sum;
> >  
> > -	if (migrated)
> > +	if (migrated || created)
> >  		attach_entity_load_avg(cfs_rq, se);
> >  
> > -	if (decayed || migrated)
> > +	if (decayed || migrated || created)
> >  		update_tg_load_avg(cfs_rq, 0);
> >  }
> 
> How much extra code gets generated for this? These _are_ hot paths.

Okay, you are right. It's a hot path.

> 
> > @@ -4136,6 +4137,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> >  	struct cfs_rq *cfs_rq;
> >  	struct sched_entity *se = &p->se;
> >  
> > +	flags = flags | (xchg(&p->migrated, 0) ? ENQUEUE_MIGRATED : 0);
> 
> Yeah, no way. xchg() is an absurdly expensive instruction, we do not
> place that unconditionally in the enqueue path.

Okay.

> 
> > @@ -5021,7 +5023,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int next_cpu)
> >  	remove_entity_load_avg(&p->se);
> >  
> >  	/* Tell new CPU we are migrated */
> > -	p->se.avg.last_update_time = 0;
> > +	p->migrated = 1;
> >  
> >  	/* We have migrated, no longer consider this task hot */
> >  	p->se.exec_start = 0;
> > @@ -8082,7 +8084,7 @@ static void task_move_group_fair(struct task_struct *p)
> >  	set_task_rq(p, task_cpu(p));
> >  
> >  #ifdef CONFIG_SMP
> > -	/* Tell se's cfs_rq has been changed -- migrated */
> > +	/* Tell se's cfs_rq has been changed */
> >  	p->se.avg.last_update_time = 0;
> >  #endif
> >  	attach_task_cfs_rq(p);
> 
> So my tiny little patch removed more code than it added, and simplified
> a few things, like the above. Now we have 2 states to worry about.
> 
> How is this making things better?

As I said, this patch is mainly for 3/3 patch. But if you worry about
regression by this 1/3 patch, I will think more about another way.

> 
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index af6f252..66d0552 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1158,6 +1158,7 @@ static const u32 prio_to_wmult[40] = {
> >  #define ENQUEUE_WAKING		0
> >  #endif
> >  #define ENQUEUE_REPLENISH	8
> > +#define ENQUEUE_MIGRATED	16
> 
> Won't actually apply that..

Okay, I got your concern, let me think more..

By the way, what do you think about the approach of 3/3 patch?

Thanks,
Byungchul

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